This invention relates to an apparatus for detecting the location of a defective or failed fuel assembly containing a failed fuel pin in a nuclear reactor. More particularly, the present invention relates to a failed fuel location detector utilizing a combination of a tag gas system and a selector valve system that have been employed independently in the past.
Conventional failed fuel location detectors for use in a fast breeder reactor can be classified into tag gas systems and selector valve systems. In the tag gas system, tag gas (tracer gas) consisting of rare gases such as Xe, Kr or the like, that are inert to irradation, is sealed in a capsule or the like, the capsule is introduced into each of a plurality of fuel pins constituting a fuel assembly and the tag gas is released into the fuel pin by subsequently opening each capsule. In this case, several kinds of rare gas isotopes are mixed at various ratios (tag gas ratios) and the isotope ratios are varied so that each of the fuel assemblies can be identified by a specific isotope ratio. If a certain fuel pin fails, the tag gas is released from the failed fuel pin together with fission product gases, and these gasses move into a cover gas space within a reactor vessel through liquid sodium coolant circulating inside the reactor vessel. Since the tag gas is diluted by the cover gas to the order of parts per billion (ppb), it is concentrated up to a concentration necessary for analysis and then subjected to mass spectrometer measurements. The gas ratio thus detected is input to a computer so that the calculated ratio is compared with the predetermined ratios to determine the location of the defective fuel assembly containing the failed fuel pin.
On the other hand, the selector valve system utilizes a selector valve device as shown diagrammatically in FIGS. 1 and 2. In the selector valve device 1, a thin sampling pipe 2 is disposed at a coolant outlet of each fuel assembly (not shown) so as to separately sample part of liquid metal coolant flowing out from each fuel assembly. The ends of these sampling pipes 2 are gathered at one position inside the reactor and are arranged so as to open circumferentially at the periphery of a sliding plate 3. A rotary pad 6 is slidably provided on the sliding plate 3 and is rotated by a motor 5 via a rotary shaft 4. The rotary pad is connected to a delayed neutron detector 9 by a tube 7 via an electromagnetic pump 8. As the rotary pad 6 rotates slidably on the periphery of the sliding plate 3, a particular sampling pipe 2 opened at the periphery of the sliding plate 3 is selected and part of the liquid metal coolant flowing out from the corresponding coolant outlet of the fuel assembly is sampled. When a fuel failure occurs, the fission products are emitted into the coolant. The fission products contain delayed neutron precursors that emit delayed neutrons to disintegrate. These delayed neutrons produced as a result of the decay of the precursor are detected by the delayed neutron detector 9, thereby determining the location of the failed fuel assembly.
Both of these systems has merits and demerits, but in a large nuclear reactor on a commercial scale, the number of fuel assemblies employed is 300 or more so that the systems inevitably become large and complicated.
In the tag gas system, for example, as the number of fuel assemblies becomes larger, more kinds of tag gases are needed and the absolute quantity of each of the tag gas nuclides becomes smaller necessarily. Accordingly, the concentrator necessary for the analysis of the tag gas must have a higher performance, and the analyzer must have a higher sensitivity. Thus, not only does the system become complicated and expensive as a whole, but also a longer period of time is necessary for the detection of the failed fuel assembly.
In the selector valve system, on the other hand, the system becomes vary large in size if the number of fuel assemblies exceeds 300 and the space must be secured for this system in addition to the essential space required for an upper core structure of the reactor. Since the upper core structure is thus restricted in space, it inevitably becomes complicated in construction. Moreover, since a large load is applied to a point on the rotary pad of the selector valve device, another problem develops in conjunction with the resistance to earthquakes.